1. Field
This disclosure relates generally to non-volatile memory devices, and more specifically, to programming a non-volatile memory device.
2. Related Art
Non-volatile memory (NVM) is the general term used to describe the type of memory that retains its data even when power is turned off, and this sort of memory is typically used to store data that must not be lost when a device incorporating the memory loses power. Such devices include computers, CD-ROMs, mobile phones, digital cameras, compact flash cards, mp3 players and Micro-Controller Units (MCUs) from the automotive, aero and other industries.
Types of non-volatile memory include Read Only Memory (ROM), Electrically Programmable Read Only Memory (EPROM), Electrically Erasable and Programmable Read Only Memory (EEPROM), Flash EEPROMs, Non-Volatile Static Random Access Memory (NVSRAM), Ferroelectric Random Access Memory (FeRAM), and the like.
Memory cells store information by storing charge on an insulated piece of semiconductor material, known as the floating gate. Typically, the insulating material is a layer of Silicon Dioxide. The charge is moved onto the insulated material forming the memory cell by either Hot Carrier Injection (HCI) or Fowler-Nordheim Tunnelling (FNT). Each individual memory cell can store a single bit of information, thus they are often referred to as bitcells.
For non-volatile memory (NVM) operation, the most popular programming method is hot carriers injection (HCI) in a memory cell. HCI works by applying a high voltage on the gate and a high voltage bias across the channel of the memory cell, resulting in the “heating” and impact ionization at the high electric field region, i.e. energy injection, of the carriers within the channel, which provides some of the carriers with enough energy to surmount the silicon dioxide energy barrier, and thus are “injected” into the floating gate. When there is little or no charge on the floating gate, the threshold voltage Vt of the transistor forming the memory cell is low. As charge is moved onto the floating gate during programming, the threshold voltage Vt of the memory cell increases. Once the threshold voltage reaches a predetermined level, the memory cell is considered programmed.
The presence of the high-energy “hot” carriers causes physical damage to a tunnel oxide insulation region in the memory cell between the drain electrode and a floating gate. As the number of program and erase cycles increases, the damage to the oxide propagates from the drain electrode toward the source electrode, thus degrading the efficiency of programming, read, and erase operations. The deterioration increases over programming cycles and can eventually cause the memory cell to fail as key parameters such as threshold voltage shift. As next-generation devices are developed, the size of the available tunnel oxide region is decreasing while the number of cycles per lifetime of the device is increasing.